In operation of an electronic device that includes electronic components, such as semiconductor chips, the electronic components generate heat as electronic circuits are energized. The amount of heat generated during the operation increases with increasing power of an electronic device. An excessively high temperature may alter the characteristics of a semiconductor chip, resulting in unstable operation of the electronic device. Furthermore, an excessively high temperature for a long period of time may alter the quality of a bonding material (for example, solder) or an insulating material (for example, a synthetic resin) of an electronic component, resulting in failure of an electronic device. It is therefore necessary to rapidly dissipate heat generated by an electronic component. Thus, various techniques for dissipating heat with a heat-release material have been investigated.
A semiconductor chip may be directly fixed to a heat-release material. Alternatively, a semiconductor chip may be soldered or brazed to an aluminum nitride (AlN) substrate having directly-bonded Al electrodes (wherein the semiconductor chip being soldered or brazed to the al electrodes) (so-called DBA substrate), and then be fixed to a heat-release material by soldering or brazing. A heat-release material to be fixed with a DBA substrate must have a thermal expansion coefficient close to that of the DBA substrate, which is in the range of 5 to 7×10−6 K−1. As currently used heat-release materials, W—Cu based composite materials have a thermal expansion coefficient in the range of 6 to 9×10−6 K−1, and Mo—Cu based composite materials have a thermal expansion coefficient in the range of 7 to 14×10−6 K−1. A thermal expansion coefficient close to that of a target material can reduce the effect of thermal stress caused by a heat generated by semiconductor chip.
Furthermore, a heat-release material fixed to a DBA substrate is generally bonded to a dissipating fin (for example, formed of Al or Cu) by solder, brazing or with electroconductive grease.
A heat-release material requires a high thermal conductivity, as well as a low thermal expansion coefficient. However, it is difficult to achieve both of them at the same time. In many cases, therefore, a composite material composed of a material having a low thermal expansion coefficient and a material having a high thermal conductivity is used.
For example, Japanese Examined Patent Application Publication No. 5-38457 proposes metal-metal based composite materials, such as W—Cu and Mo—Cu. This technique utilizes a low thermal expansion coefficient of W or Mo and a high thermal conductivity of Cu.
Japanese Unexamined Patent Application Publication No. 2002-212651 discloses ceramic-metal based composite materials, such as SiC—Al and Cu2O—Cu.
In these techniques, the thermal expansion coefficient is generally limited by the law (or rule) of mixtures (described below). More specifically, the thermal expansion coefficient of a composite material is in the vicinity of the volume average of the thermal expansion coefficients of component materials. Furthermore, in the case of W—Cu and Mo—Cu materials, W and Mo are rare metals and are targets for speculation. This increases the raw material costs and may cause shortages thereof. Furthermore, W—Cu and Mo—Cu materials require hot working. SiC—Al materials require difficult machining and reduction. Cu2O—Cu materials also require hot working. Thus, it is difficult to produce a plate material by an inexpensive method. A further improvement is needed.
Japanese Unexamined Patent Application Publication No. 2000-239762 and “Development of Cu—Cr Multiphase Alloy,” The Furukawa Electric Co., Ltd., Report, Vol. 107, January, 2001, pp. 53-57 disclose a technique for achieving a low thermal expansion coefficient and a high thermal conductivity in metal-metal composite materials, such as Cr—Cu and Nb—Cu. These documents disclose a technique in which a Cu alloy containing 2% to 50% by mass of Cr is shaped by a melting and casting method is subjected to hot working to produce generally spherical primary crystalline Cr phases of an ingot structure, and is further subjected to cold working (for example, paragraph [0014] in Japanese Unexamined Patent Application Publication No. 2000-239762) to adjust the aspect ratio of the Cr phases to be at least 10. These documents say that the technique can achieve a thermal expansion coefficient lower than that expected by the law of mixtures.
However, according to this technique, the thermal expansion coefficient is reduced only just by about 10% relative to that expected by the law of mixtures when the aspect ratio is as high as 100 of more ([0014] cited above). Adjusting the aspect ratio of the (generally spherical) Cr phases, which are primary deposited phases in a solidification process, to be 100 or more may require cold rolling at a reduction of about 90% or more.
In a melting and casting method, an increase in Cr content results in an increase in melting point. In addition, segregation during solidification makes it difficult to produce a homogeneous alloy. Actually, it is impractical to melt and cast a material containing more than 30% by mass of Cr and achieve an aspect ratio sufficient to greatly reduce the thermal expansion coefficient by cold working. Actually, “Development of Cu—Cr In-Situ Composite,” The Furukawa Electric Co., Ltd., Report, Vol. 107, January, 2001, pp. 53- 57 and the examples of Japanese Un-examined Patent Application Publication No. 2000-239762 do not disclose an example containing more than 30% by mass of Cr.
Furthermore, a hot-forging or hot-rolling process, as well as homogenization heat treatment at a high temperature for a long period of time, is necessary to homogenize an alloy formed by the melting and casting method. This increases the manufacturing costs, and limits the dimensions of a heat-release material serving as a product.
Siemens Forsch.-Berd. Bd, 17, 1988, No. 3 discloses a technique for producing a homogeneous Cr—Cu alloy containing at least 30% by mass of Cr by melting and cold working. More specifically, a round bar is produced by casting utilizing an expensive arc-melting method (a melting and casting method utilizing arc discharge) using a sintered powder mix of Cr and Cu as a consumable electrode, and by extrusion to facilitate the deformation of Cr, which has insufficient ductility at room temperature. In the extrusion process, hydrostatic pressure is applied to Cr via a Cu matrix, thus facilitating the processing of Cr. This technique has a problem with economical efficiency, and is not suitable to produce a thin plate material, such as a heat-release material.
In Japanese Unexamined Patent Application Publication No. 2005-330583, as a technique for adapting a Cr—Cu material to a heat-release material, we disclose a technique for improving the thermal expansion coefficient of the Cr—Cu material by precipitating fine particulate Cr phases having a major axis of 100 mm or less from a Cu matrix by aging heat treatment. Among others, in a powder metallurgy process, a Cr powder is used for Cr—Cu alloying and making composite by sintering or infiltration, and particulate Cr phases are precipitated from a Cu matrix by aging heat treatment, as described above.
It has heretofore been difficult to fully accomplish the task of producing a thin plate material having a low thermal expansion coefficient and a high thermal conductivity. For example, the technique described in Japanese Unexamined Patent Application Publication No. 2000-239762 and “Development of Cu—Cr Multiphase Alloy,” The Furukawa Electric Co., Ltd., Report, Vol. 107, January, 2001, pp. 53-57 is limited by processability and the Cr content. The technique described in Siemens Forsch.-Ber. Bd, 17, 1988, No. 3 needs considerable costs, even though a material can be processed.
While the technique described in Japanese Unexamined Patent Application Publication No. 2005-330583 is relatively excellent, there is a problem concerning the production of a thin plate material. Furthermore, according to that technique, Cr phases are randomly precipitated in three dimensions. Thus, the material has the same expansion coefficient in any direction. A heat-release material for semiconductors often has a thin plate shape. Thus, when such a heat-release material is joined to a semiconductor, the difference in thermal expansion coefficient must be reduced in in-plane directions. We have a view that the technique still leaves room for improvement.
Furthermore, that technique achieves a low thermal expansion coefficient only by controlling the state of the precipitated phase. Thus, depending on the condition for soldering or brazing between the heat-release material and a DBA substrate, for example, soldering or brazing at a high temperature beyond the aging temperature for a long period of time, the precipitated phase may change. This alters the characteristics of the heat-release material. Thus, it is likely that the heat-release material does not stably have a low thermal expansion coefficient.
It could therefore be advantageous to provide a Cr—Cu alloy having a low thermal expansion coefficient particularly in in-plane directions even after soldering, a high thermal conductivity, and excellent processability. It could also be helpful to provide a method for producing the Cr—Cu alloy. It could still further be helpful to provide a heat-release plate for semiconductors and a heat-release component for semiconductors, each comprising the Cr—Cu alloy.